1. Field of the Invention
The present invention relates generally to a throttle control system for an automotive internal combustion engine for controlling a throttle valve angular position in order to adjust an engine output torque for suppressing wheel slippage and for better tread/road traction. More specifically, the invention relates to a throttle control system of a tandem throttle type air induction system of the internal combustion engine, which has a pair of throttle valves arranged in an air induction passage in tandem fashion.
2. Description of the Background Art
In the recent years, there have been proposed various traction control systems for adjusting driving torque to be transferred to driving wheels of automotive vehicles so as to minimize wheel slippage and obtain maximized or optimal driving performance as well as driving stability.
Among variations in the recently proposed traction control systems, some systems have tandem throttle layout for arranging pair of throttle valves in tandem fashion. One of the throttle valves in such type of throttle valve layout, which serves as a primary throttle valve, is associated with an accelerator, such as an accelerator pedal, so that it may vary open angle according to operational magnitude of the accelerator. The other throttle valve which serves as a secondary throttle valve, is associated with a traction control actuator so that the open angle of the secondary throttle valve can be controlled for adjusting the engine output in the wheel slippage suppressive purpose.
In such type of the traction control system, it is essential to obtain information concerning the angular position of the secondary throttle valve with high accuracy so as to achieve high precision in traction control. Namely, due to tolerance in production of the throttle valve or error in assembling, there may be contained a certain magnitude of error in a signal representative of the angular position of the secondary throttle valve. Such error contained in the secondary throttle angle indicative signal will affect for precision level of traction control.
Furthermore, in the prior proposed traction control systems, the secondary throttle valve is placed at minimum open angle position close to fully closed position when wheel slippage is substantial or when the road friction is substantially low. At such secondary throttle valve condition, the amount of intake air to be introduced into the combustion chamber of the engine becomes minimum. If the minimum intake air amount is determined in view of the normal engine temperature condition, possibility of engine stalling during cold engine state becomes substantially high. On the other hand, when the minimum intake air flow amount is set in view of the cold engine state, reduction of the engine output at normal engine temperature condition becomes too small to effectively recover tread/road traction.